在二叉搜索树中找最小的大于某个key值的节点

如
         8
      /       \

   6         12
  /             /   \
2          11    14

key = 8 返回11
key = 1 返回2
key = 16 返回NULL

struct TreeNode
{
	int val;
	TreeNode* left;
	TreeNode* right;
};

// 迭代实现
TreeNode * FindCeiling(TreeNode *root, int key)
{
	TreeNode * ceiling = NULL;
	TreeNode * current = root;
	while(current)
	{
		if(current->val <= key)
			current = current->right;
		else
		{
			ceiling = current;
			current = current->left;
		}
	}
	return ceiling;
}

// 递归实现
TreeNode * FindCeiling(TreeNode *root, int key)
{
	if(root == NULL)
		return NULL;
	if(root->val <= key)
		return FindCeiling(root->right, key);
	else
	{
		TreeNode *ceiling = FindCeiling(root->left, key);
		return ceiling ? ceiling : root;
	}
}